In the past swimming pools of one type or another have all been required to be drained periodically for maintainance or cleaning. In northern climates draining is usually required during the winter months to prevent freeze damage.
Such draining has generally required the use of pumps which entails a considerable expense and access to a source of power which may not be readily accessible. For swimming pools that are raised above the ground and are supported upon the ground level such added expense may make the cost unacceptable and the absence of a source of power for a pump may greatly add to the inconvenience of draining to the pool.